1. Field of the Invention
This invention relates to a method for fabricating a gas diffusion electrode assembly for fuel cells. More specifically, it relates to a method for fabricating a gas diffusion electrode assembly for proton exchange membrane fuel cells comprising gas-diffusible electrodes and a cation exchange membrane through which protons migrate.
2. Prior Art
Fuel cells of solid polymer electrolyte type utilizing cation exchange membranes as electrolyte have been developed as the forth generation type of fuel cell following the ones of phosphoric acid type, molten carbonate type and solid electrolyte type. Various developments as to the fuel cells of this type are being conducted because they can show higher electromotive force per unit area and hence they can be made smaller as compared with the precedent fuel cells.
Fuel cells of this type are usually called "Proton Exchange Membrane Fuel Cells" (PEMFC's) and have a structure shown in the appended FIG. 1. Generally, perfluorosulfonic acid type cation exchange membranes are considered suitable for the solid polymer electrolyte, which is the most important component of PEMFC, because of their durability, chemical resistivity, oxidation resistance, thermal stability and so on. However, it is still important to select an appropriate combination of the proeperties of cation exchange membrane and electrode catalyst layers to be bonded to the membrane so that well-balanced cell characteristics can be obtained.
Hitherto, as the cation exchange membrane for PEMFC's, mainly used are commercially available perfluorosulfonic acid type of cation exchange membranes such as NAFION membranes available from E. I. DuPont Nemours and Co., U.S.A. such as NAFION 117, which has protons on ion-exchange groups (H-form) and has an ion-exchange capacity of 0.909 meq/g of dry resin and a membrane thickness of about 178 .mu.m. However, when these commercially available membranes are used in PEMFC's, thicknesses of these membranes are generally too large to obtain acceptably low electric resistivity of the membrane and, as a result, cell voltage is low. In particular, when air is fed to the anodes of PEMFC's, high cell voltage cannot be obtained by these membranes and hence this drawback of these membranes is a serious problem in practical applications of PEMFC's.
On the other hand, because PEMFC's have electrode catalyst layers on the cation exchange membrane as shown in FIG. 1, it is important that both the electrode catalyst layers and properties of the cation exchange membrane are sufficiently integrated so that both components, i.e., the cells, can show sufficient characteristics. Therefore, the method for bonding the cation exchange membrane and the electrode catalyst layers is also important.
Conventional methods for bonding a cation exchange membrane and an electrode catalyst layer or forming an electrode catalyst layer on a cation exchange membrane include a method comprising uniformly applying a preliminarily provided mixture containing electrode catalyst loaded on fine carbon particles and a polytetrafluoroethylene dispersion as a binder on aluminum foils, drying the mixture to form electrode catalyst layers and bonding the resulted electrode catalyst layers on both surfaces of cation exchange membranes of H-form (having protons (H.sup.+) as the cations of the ion exchange groups) by hot pressing; and a method comprising directly applying said mixture on the cation exchange membrane, drying the mixture and subjecting the membrane having the dried layer to hot pressing. However, in these conventional methods, it is rather difficult to establish the conditions of the hot pressing, because the softening temperature of the polytetrafluoroethylene is quite higher than that of H-form perfluorosulfonic acid membrane and because it is necessary to vary the hot pressing temperature depending on the mixing ratio of the fine carbon particles and the polytetrafluoroethylene emulsion. Furthermore, the electrode catalyst layers formed by these method are likely to become thick, and hence the obtained PEMFC's are likely to be expensive because noble metal such as platinum is usually used for the catalyst.
It is also possible, when H-form cation exchange membranes such as the above-mentioned NAFION 117 are used, to form electrode catalyst layers on the cation exchange membranes by preparing a mixture of a solution of H-form of perfluorosulfonic acid copolymer similar to that of the cation exchange membranes and catalyst loaded on fine carbon particles, uniformly applying the mixture on either or both surfaces of the cation exchange membrane, drying the mixture and subjecting the dried layers to hot pressing together with the membrane in a manner similar to the method described above. However, because the softening temperature of the H-form cation exchange membrane is rather low, it is necessary to carry out the hot pressing at a quite low temperature, for example, at a temperature lower than about 140.degree. C. The hot pressing at such a low temperature leads to insufficient bonding of the electrode catalyst layers and the cation exchange membranes.
Therefore, there is still a need for an improved method for bonding the cation exchange membrane, electrode catalyst layers and carbon cloth or paper in the fabrication of PEMFC's in view of characteristics and cost of the cells, efficiency of the method and the like.